The present invention relates generally to a memory device utilized in connection with a page printer which forms an image on a piece of paper by scanning a photosensitive drum with a light beam modulated in accordance with print data, and, in particular, to a single port memory device utilized with such a page printer.
Conventional page printers form images utilizing the principles of xerography. Such printers are capable of printing both characters and figures with a very high quality. To print both characters and figures on the same page in a mixed manner, data for printing characters and the data for printing images must be superimposed upon each other.
To superimpose characters, the page printer includes a memory and central processing unit ("CPU") as shown in FIG. 1. A CPU 200 operates to control the memory based upon a control program. A program ROM 201 stores the program for controlling printing steps. A font ROM 202 stores character pattern data. A memory control circuit 203 is coupled to CPU 200 and couples a two port image processing memory 204 to CPU 200. Image processing memory 204 stores image data so that print data can be processed in a bit by bit manner. A work memory 205 is also coupled to CPU 200 through memory control circuit 203. A parallel series converter 206 receives the data from imaging processing memory 204 and converts the parallel input into a bit unit output to be utilized by the printer.
When image data is stored in image processing memory 204, the character pattern data is read from font ROM 202 by CPU 200 and input to image processing memory 204 by memory control circuit 203. The image data and character pattern data are superimposed upon each other at high speeds by image processing memory 204. Further, the image processing memory is capable of processing the data in individual bit units and can shift the print data form in image processing memory 204 in bit units to finely adjust the printing positions at high speeds.
The conventional memory allows the print data to be processed in various ways. However, the image processing memory must be a two port memory having an input port and an output port which act independently of each other. Therefore, an expensive two port memory is required increasing the cost of the device. Further, the work memory, which is a one port memory, is of a different configuration than the image memory. This requires that the image processing memory and the work memory be operated as independent memories. Therefore, memory capacity which is not being used in either of the work memory or image processing memory can not be shared between the two memories, decreasing overall efficiency.
The conventional page printer forms images using very small dots enabling part of the image print data to be read and copied into another memory region within the overall memory. Such a copying process requires processing which does not allow the data to be affected by the data presently in the memory region being copied into the other region. Since the processing is carried out in software manner using a CPU, an extended period of time is required for the copy process preventing high speed printing performance of the page printer.
Accordingly, it is desired to provide a memory for a page printer which overcomes the disadvantages of the prior art devices described above and is capable of using a one port memory.